This invention relates to an electromagnetic clutch.
Generally, an electromagnetic clutch is constructed as shown in FIG. 20. In FIG. 20, indicated at 100 is an electromagnetic clutch used in a copy paper feeding mechanism of a copying machine and the like. The electromagnetic clutch 100 has a field member 102 formed with a cylindrical hollow 101 along an axis of the field member 102. A hollow shaft 103 is rotatably inserted in the hollow 101. The field member 102 is provided a bobbin 104 therein. The bobbin 104 is wound with a field coil 105.
A rotor 107 is fixedly secured to the hollow shaft 103. The rotor 107 is formed with a number of through holes. A driving member 109 formed with teeth is rotatably mounted on the hollow shaft 103. Further, an armature member 108 is attached to the driving member 109 by way of a disc spring 110. The armature member 108 is rotatable with respect to the hollow shaft 103.
The hollow shaft 103 is formed with grooves 111, 112 at both ends thereof. Fastening ring members 113, 114 are fitted in the grooves 111, 112 to hold the field member 102, the rotor 107, and the driving member 109 from slipping off from the hollow shaft 103. Among the parts constituting the electromagnetic clutch, at least the field member 102, the hollow shaft 103, the rotor 107, and the armature member 108 are made of a material having magnetism, such as ferrous material.
The thus constructed electromagnetic clutch 100 is operated as follows. The driving member 109 is rotated about the hollow shaft 103 driven by an unillustrated drive source. Specifically, when an electric current is flowed in the coil 105, magnetic flux generates along the field member 102, the hollow shaft 103, and the rotor 107. The armature member 108 is attracted onto the rotor 107 by leakage flux passing through the holes of the rotor 106 against a biasing force of the spring 110. Accordingly, the rotor 107 is rotated together with the driving member 109, and the hollow shaft 103 is also rotated together with the rotor 107 since the rotor 107 is fixedly secured to the hollow shaft 103.
On the other hand, when the supply of electric current is stopped, the coil 105 is deenergized, and the flux disappears. Hence, the magnetic attraction force of the rotor 107 also disappears. As a result, the armature member 108 is returned to the initial state where the armature member 108 is biased toward the driving member 109 by the biasing force of the spring 110 and is away from the rotor 107. Consequently, although the driving member 109 is rotating, the hollow shaft 103 does not rotate further.
The hollow shaft 103 is formed with a hollow along an axis of the hollow shaft 103. As shown in FIG. 21, the hollow formed in the hollow shaft 103 has a D-shaped hollow section in an end of a portion over which the field member 102 is provided. In the hollow of the hollow shaft 103 is inserted a shaft constituting the copy paper feeding mechanism. The shaft has in one end thereof a D-shaped section in correspondence to the D-shaped hollow section of the hollow shaft 103. With this arrangement, the rotational force of the hollow shaft 103 of the clutch 100 is transmitted to the shaft of the copy paper feeding mechanism. The D-shaped section engagement ensures an integral rotation of the hollow shaft 103 and the shaft of the copy paper feeding mechanism.
It has been cumbersome to produce the hollow shaft 103 by machining a metal rod due to the presence of the D-shaped hollow section. For this reason, the hollow shaft 103 has been produced by filling powder having magnetic particles in a mold having a molding space in agreement to the hollow shaft 103, and sintering it at a high temperature, and forming the grooves 111, 112 in two ends of the sintered shaft by machining.
However, such hollow shaft production method requires many processes, such as molding and sintering. Further, the shaft inevitably suffers a deformation in the sintering process due to an irregularity in the density of powder in the molding process. Accordingly, it has been necessary to grind the sintered shaft into the specified shape. The production cost of hollow shafts has been considerably high. This has inevitably increased the whole production costs of electromagnetic clutch.
In the conventional electromagnetic clutch 100, the rotor 107 is constructed as shown in FIGS. 22 and 23. FIG. 22 is a front view of the rotor 107, and FIG. 23 is a side view in section of the rotor 107.
The rotor 107 is formed with a through hole 1071 in a center thereof, a circular recess 1072 in one side thereof, and an annular groove 1073 in the same side. The annular groove 1073 has a greater depth than the circular recess 1072. The terminal end wall of the annular groove 1073 is formed with through holes 1074, 1075 and 1076, each having the form of an arc.
In other words, the rotor 107 is coaxially formed with an inner annular projection 1077 and an outer annular projection 1078 on the one side. The inner and outer annular projections 1077 and 1078 are formed in a concentric manner. The side of the rotor 107 where the projections 1077 and 1078 are formed faces the armature member 108, while the other side of the rotor 107 faces the field member 102. With this construction, when the coil 105 is energized, the armature member 108 is attracted on the rotor 107 by leakage magnetic flux passing through the holes of the rotor 106.
The rotor 107 has the circular recess 1072, annular groove 1073, through holes 1074 to 1076, and is thus very complicated in construction. This has made it difficult to produce the rotor 107 at one time by press molding. Accordingly, the rotor has been produced by machining. However, the machining of the rotor needs a long time, which consequently makes it difficult to produce a large quantity of rotors in a short time, and to reduce the production cost of rotors. This has inevitably increased the whole production costs of electromagnetic clutch.
For electromagnetic clutches, further, there has been the demand that when the field coil is deenergized, the hollow shaft always stops and keeps at a fixed position. For example, in the case where an electromagnetic clutch is used in a copy paper feeding mechanism, an error in the stop position results in a failure in the copy paper feeding, e.g., feeding copy paper beyond a predetermined position. Also, if the stop position is not maintained, there is a likelihood that copy paper is not reliably nipped by next roller pairs, e.g., registration roller pair which performs a secondary feeding operation.
To solve this problem, the electromagnetic clutch may be provided with a braking member. However, the provision of a braking member causes the construction and production more complicated, which accordingly makes it difficult to reduce the production costs of electromagnetic clutch.